But by that same token, hundreds of PSDs are posted on Dribbble every day. As Allan, the designer for Designer News and Layervault, said when I asked him if he wanted me to take the resources down, he said 'Why? It's wonderful'.

That is Allan's right, and Heroku's right. But Allan can only speak for himself, Heroku can only speak for Heroku. So while it's good that you've asked them for their opinion, they have no agency and confer no expectations as to what MailChimp does or does not do.

You're not really giving people a choice if you expect only one answer when you ask them for permission, you know. ;)